1. Field of the Invention
The present invention relates to an apparatus and method for controlling a tracking error balance in an optical disc apparatus.
2. Description of the Related Art
After Compact Discs (CDs), that is, optical recording media capable of recording 74 minutes of music or about 650 Mbytes of data, have been commercialized, Digital Versatile Discs (DVDs) capable of recording a 2-hour Standard Definition (SD)-class movie were widely commercialized, and Blu-ray Discs (BDs) and High-Density Digital Versatile Discs (HD-DVDs) capable of recording a High Definition (HD)-class movie will appear on the market in the near future.
Optical recording media, such as CDs, DVDs and BDs, are disc-shaped media for recording data using optical characteristics, and data can be written on the discs and read from the discs using optical pickups. Optical recording media include discs for playing on which data is written already, and discs for recording, such as CD-R/RW, DVD-R/+R/-RW/+RW/RAM and BD-R/-RE media, that can be written or rewritten.
An optical pickup writes data on a disc or reads data from a disc in the state in which a laser beam is correctly focused on the track of the disc, which is rotating at high speed.
Such an optical pickup is provided with optical components such as an objective lens and a beam splitter, instrumental components such as an actuator and a base, and electrical components such as a Laser Diode (LD) and a photo detector, so that servo signals such as a focusing error signal and a tracking error signal, corresponding to the position error of an optical spot formed on a disc, can be obtained and the position error of the optical spot can be corrected based on the servo signals. As such, a servo operation can be performed.
In the detection of a focusing error signal, an astigmatism method is generally used regardless of the type of disc and the purpose of the disc, such as writing and playing. In the detection of the tracking error signal, a 3-beam method and a Differential Phase Detection (DPD) method are generally used for discs for playing, and a Differential Push-Pull (DPP) method is typically used for discs for writing.
FIG. 1 is a view illustrating the principle by which a tracking error signal is detected using the DPP method according to a related art.
The DPP method is an improvement on the related art 1-beam push-pull method. This method is one capable of removing offset occurring due to the movement of the objective lens in a radial direction, or due to the tilt of a disc, and detecting a stable tracking error signal.
In the DPP method as shown in FIG. 1, when a laser beam output from a light source via a diffraction element, called a “grating,” is separated into 0-order and +/−1-order diffracted light and a main beam (that is, the 0-order diffracted light) is radiated on the groove of the track of a disc, the grating is controlled such that sub beams (that is, the +/−1-order diffracted light) are radiated on a land adjacent to the groove where the main beam is located. That is, the sub beams are spaced apart a ½ track pitch from the main beam. At racking error signal is detected based on a difference signal between the right and left of each beam on the basis of the radial direction.
The main beam reflected from the disc is received by a main photo detector segmented into four (a, b, c and d), and is detected as a Main Push-Pull (MPP) ((A+D)−(B+C)) signal, that is, a push-pull signal. Each sub beam reflected from the disc is received by a sub photo detector that is segmented into two (E1, E2) or (F1, F2), and is detected as a Sub Push-Pull (SPP) ((E1−E2)+(F1−F2)) signal. When the main beam and the sub beam are spaced apart a ½ track pitch from each other, the MPP and SPP signals have opposite phases.
Both the MPP and SPP signals experience offset in the same direction due to a tilt or movement of the objective lens in the radial direction. Accordingly, a push-pull signal from which the offset has been removed can be obtained through calculation based on DPP=MPP−k×SPP where k is a proportional constant.
Improved playing and writing performance can be obtained only when amplitude, offset and balance are accurately controlled for servo signals, such as a focusing error signal and a tracking error signal. The balance control of the servo signals, particularly the balance control of the tracking error signal, causes the beam spot to accurately follow the center of a track by making upper and lower amplitudes similar, as illustrated in FIG. 2.
In the DPP method, the control of tracking error balance is adapted to control the gain for the MPP signal so that (A+D) and (B+C) having the same signal level are output, and to control the gain for the SPP signal so that the signal levels of (E1+F1) and (E2+F2) are also the same.
When an optical disc is played, an operation of controlling tracking error balance is generally performed and a controlled value is generally used unchanged in both recording and playing operations. However, if tracking error balance controlled for a playing operation is applied unchanged to recording operations, the writing quality is affected to some degree due to the difference in the optical output at the time of playing from and writing to the disk. Therefore, it is advantageous if the optical output is performed in conditions similar to the writing conditions, in particular, in conditions where the optical output is similar to the output power (10 to 20 mW). In other words, in order to improve the writing quality, the tracking error balance can be controlled under conditions similar to those of the writing operation, that is, in a state in which the output power of the LD is increased to about 2 mW (the output power for playing is usually about 1 mW).
If the tracking error balance for writing is not normally controlled under the above conditions, the tracking error balance control operation is performed while the output power of the LD is gradually increased, for example, the output power value of the LD is gradually increased from 2 mW to 5 mW.
However, if the tracking error balance is controlled while the output power is increased as described above, data can actually be recorded on the optical disc, thus resulting in the damaged disc, which is a problem.